Demographic effect of local breeding environment on Tree swallow Tachycineta bicolor

Background/Question/Methods

Aerial insectivores have shown large declines in abundance across North America, with intensification of agricultural practices often identified as one of the main cause of decline. However, the environmental variables and the vital rates influencing population dynamics are not always consistent for migratory birds and differ between populations. Moreover, human driven environmental changes, can have direct and indirect effect on wild population and only a few studies have explored their combined effects on wild populations. Using a long-term dataset of individually marked Tree swallows (Tachycineta bicolor) monitored in 400 nesting boxes over 13 years (2006-2019) in southern Québec, Canada, we used multi-state CMR models to integrate and quantify the effects of local climatic variables, density of inter-specific nest competitors and agricultural intensification on the dynamics of this population.

Results/Conclusions

Population growth rate was highly dependent of immigration rate and adult survival with little variation in the elasticities and sensitivities caused by annual variation. We found a strong negative effect on the success of reproduction of the nest-competition with House sparrow (Passer domesticus), which appeared to be a better predictor of the population dynamics than the direct effect of agricultural intensification. Most of the temporal variation in the population’s dynamics, however, remained unexplained, suggesting that carry-over effects from migration and wintering grounds may be just as important for this population as the local conditions it experiences on its breeding grounds. Knowledge about demographic processes and the environmental predictors driving them, is essential for a better understanding of direct and indirect impact of agriculture on population dynamics in rural habitat.